TocToc TocToc - 1 year ago 237
C# Question

Case insensitive access for generic dictionary

I have an application that use managed dlls. One of those dlls return a generic dictionary:

Dictionary<string, int> MyDictionary;

The dictionary contains keys with upper and lower case.

On another side I am getting a list of potential keys (string) however I cannot guarantee the case. I am trying to get the value in the dictionary using the keys. But of course the following will fail since I have a case mismatch:

bool Success = MyDictionary.TryGetValue( MyIndex, out TheValue );

I was hoping the TryGetValue would have an ignore case flag like mentioned in the MSDN doc, but it seems this is not valid for generic dictionaries.

Is there a way to get the value of that dictionary ignoring the key case?
Is there a better workaround than creating a new copy of the dictionary with the proper StringComparer.OrdinalIgnoreCase parameter?

Answer Source

There's no way to specify a StringComparer at the point where you try to get a value. If you think about it, "Foo".GetHashCode() and "foo".GetHashCode() are totally different so there's no reasonable way you could implement a case-insensitive get on a case-sensitive hash map.

You can, however, create a case-insensitive dictionary in the first place using:-

var caseInsensitiveDictionary =
    new Dictionary<string, int>(StringComparer.OrdinalIgnoreCase);

Or create a new case-insensitive dictionary with the contents of an existing case-sensitive dictionary (if you're sure there are no case collisions):-

var oldDictionary = ...;
var newDictionary =
    new Dictionary<string, int>(oldDictionary, StringComparer.OrdinalIgnoreCase);

This dictionary then uses the GetHashCode() implementation on StringComparer.OrdinalIgnoreCase such that comparer.GetHashCode("Foo") and comparer.GetHashcode("foo") give you the same value.

Alternately, if there are only a few elements in the dictionary, and/or you only need to lookup once or twice, you can treat the original dictionary as an IEnumerable<KeyValuePair<K,T>> and just iterate over it:-

var myKey = ...;
var myDictionary = ...;
var item =
        x => String.Equals(x.Key,

This saves you the cost of creating a new data structure, but in return the cost of a lookup is O(n) instead of O(1).

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